When a telephone is used in an electro-acoustic speakerphone mode, acoustic coupling or “feedback” between the loudspeaker and the microphone is likely to occur. When acoustic coupling occurs, sound produced by the loudspeaker is sensed by the microphone and retransmitted back to its original source, causing undesirable artifacts, such as screeching, to be heard by the call participants.
Three common approaches for minimizing acoustic coupling are:                i. acoustic echo canceling,        ii. automatic microphone mixers, and        iii. the use of directional microphones.Some background information for each approach is now provided.        
In accordance with acoustic echo canceling, a digital adaptive filter models the acoustic coupling between conferencing system loudspeakers and conferencing system microphones (i.e., the “acoustic signature”) in the room. The audio signal that feeds the local loudspeaker is filtered using the acoustic signature stored in the adaptive filter model. This filtered signal is then subtracted from the microphone signal before the signal is transmitted to the far end, resulting in an improved signal at least under some conditions. However, acoustic echo canceling has some limitations. First, the adaptive filter might take several seconds to adapt to changes in the acoustic signature of the room. Second, adaptation can only occur when the far-end party is talking but the local parties are not. As a result, if changes to the room's acoustic signature occur during a conversation—for example, the microphone moves or a local party's body moves with respect to the local microphones—then the far-end party will hear the artifacts of inadequate cancellation until the digital filter can re-adapt.
Automatic microphone mixers can also provide an effective means to minimize loudspeaker-to-microphone coupling when a mixing algorithm operating at the near end is able to consider the audio signal that is received from the far end. In this case, audio received from the far end has the effect of attenuating the signals from local (near-end microphones when the far-end party is talking. As a result, loudspeaker-to-microphone coupling at the near end is reduced by the amount that the automatic mixer attenuates the near end's microphone signals. Automatic mixers have the advantage of providing a cleaner audio feed to the far end, not only when a single near-end microphone is in use, but also when multiple local microphones are used. However, the disadvantage with this approach is that the teleconferencing system is no longer full-duplex.
Directional microphones can also be used to reduce acoustic coupling. When a directional microphone is positioned so that its region of minimum amplitude sensitivity facing towards the loudspeaker, most or all of the sound arriving from the direction of the loudspeaker is prevented from being sensed by the microphone. However, this comes at the cost of blocking sounds produced by a call participant when the participant is situated behind the loudspeaker in relation to the microphone. The apparent reduction in microphone amplitude sensitivity with respect to some call participants makes the use of directional microphones impractical.
What is needed is a method for minimizing acoustic coupling without some of the costs and disadvantages of the prior art.